The use of noble metal catalysts in the form of gas-permeable nets is an established method of operating certain catalytic processes with optimal yields. A well-known example is the use of platinum/rhodium catalyst nets in the oxidation of ammonia for the production of nitric acid.
The net form of the catalysts affords several advantages over other catalyst configurations. For example, the net form provides for large surface development coupled with high mechanical strength. In wire drawing and wire weaving, manufacturers have established production methods for processing the catalytically active metals and metal alloys. These catalyst nets are made in web form on looms and are cut to size in circular form in accordance with the dimensions of the oxidation plants. This results in the accumulation of considerable noble metal waste which has to be reprocessed.
However, nets have the advantage that, with their loops, they form a homogeneous structure of metal wires and free throughflow openings for the reaction gases.
There are no inhomogeneities, for example in the form of seams or welds which impede uniform gas flow.
Nets made in the same way are also used in the recovery of noble metals, for example in the oxidation of ammonia. A large part of the noble metals which volatilize from the platinum/rhodium catalyst nets during the process is collected in nets of palladium alloys.
Woven nets have a number of disadvantages attributable to their production process, including the time required to set up the loom, weaving in considerable lengths and cutting out of the circular nets from rectangular webs with approximately 35% waste.
High production costs are incurred by long set-up times and the high percentage of waste and by the large amounts of expensive noble metals tied up on the loom which necessitates considerable capital outlay.
The woven nets can also be replaced by other gas-permeable configurations. For example, perforated metal foils and metal fiber nonwovens (DE-PS 15 94 716), metal felts (DE-PS 28 29 035) or folded tubular wire nets (DE-OS 22 48 811) are known.
However, these configurations have never been successfully used in practice, above all because they adversely affect the flow conditions prevailing in the reaction zone.
Knitted nets have recently been mentioned in connection with the recovery of noble metals in the oxidation of ammonia (ES-PS 0 077 121). However, no particulars of their form or production are provided.
EP-OS 0 364 153 describes a process for the production of nets of noble metal alloys, more particularly for the catalytic oxidation of ammonia and the recovery of noble metals, in which wires of noble metal alloys are knitted together with an auxiliary thread of natural fibers or a synthetic material on a knitting machine. Pure noble metal alloy wires cannot be knitted without auxiliary threads because the wires break and tear during the knitting process, the tensile strength, wire diameter, ductility and surface friction factor of the material being said to play a part. The disadvantage of including an auxiliary thread in the knitting process is that it has to be removed before the nets are used, which can be done by burning or dissolving. Both methods are too complicated and can lead to contamination of the metal alloys which can adversely affect their catalytic activity or recovery level.